This is what I found out on www.wikipedia.org/wiki/Benchmarking_(geolocating) :
Benchmarking (also known as benchmark hunting) is an activity in which individuals or teams of participants go out and find benchmarks (also known as survey markers orgeodetic control points). They typically then log their finds online. Like geocaching, the activity has become popular since about 1995, propelled by the availability of on-line data on the location of survey marks (with directions for finding them) and by the rise of hobbyist-oriented websites (for example, in the U.S., Geocaching.com).
Many survey markers in the U.S. were set over 100 years ago. There was also a surge in creating these marks in the U.S. from about 1930 to 1955, in conjunction with the expansion of map-making activities across the country. In the U.S. some of these marks (triangulation stations and GPS points, loosely also referred to as "benchmarks") have precise "adjusted" coordinates (latitude and longitude), while others, typically true elevation Bench Marks, have only coordinates scaled from a map. Some marks were established with clear descriptions on how to reach them, while others were much more poorly documented. Over time, some marks have become buried beneath the ground, or surrounded by dense woodlands. Old-time "references" (e.g., the corner of a nearby building or a distinctive tree) have sometimes been destroyed altogether. Sometimes the mark being hunted is found to have been paved over by a parking lot or blown to bits as part of a construction project.
But this diversity is often what makes people enjoy the hobby of benchmark hunting. Some hunters stick to more well documented marks, and have an easier time of it. Others go looking for more poorly described marks, and may spend hours and hours in their searches (bushwacking through difficult back country) and return to a site again and again, often without finding their quarry.